The present invention relates to a CMOS voltage divider having a first chain containing series-connected MOS transistors of a first conductivity type. Each of the transistors have identical geometrical dimensions and, at the same time, each have identical gate-source voltages, which operate in the linear range of their characteristic curve and between whose opposite ends an input voltage to be divided is present and at whose source terminals the voltage fractions can in each case be picked off.
Generally, a voltage divider circuit contains a plurality of series-connected resistance elements through which the same current flows. The divided output voltages can be picked off at the junction points of the resistance elements of the resistance chain.
If such a voltage divider circuit is intended to be used in a large scale integrated circuit, it must satisfy a number of requirements. First, an area occupied by the voltage divider circuit should be as small as possible. Second, the output voltage should depend only on the circuit geometry. Third, the quiescent current drawn by the circuit should be as small as possible. Fourth, the output resistance of such a voltage divider chain should be as low as possible in order that the circuit acts as a voltage source.
In the prior art, voltage divider circuits are known which fulfill at least some of the above requirements and use resistance elements. The resistance elements are produced either in N-type diffusion or in P-type diffusion and their sheet resistance is in the range of 10-100 ohms/unit of area. Therefore, an extremely large resistance area of the order of magnitude of 10,000 units of area is needed in order to achieve a resistance of 106 ohms which, for its part, brings about a quiescent current of just a few xcexcA. In many cases, such a large chip area is impossible or undesirable. Therefore, a voltage divider circuit of this type does not fulfill the first and third requirements.
Another possible realization of a voltage divider circuit uses MOS transistors operating in their linear range as resistance elements. The current through each transistor depends on its geometry and on its terminal voltages:
ILIN=Betaxc3x97[(Vgsxe2x88x92Vth)Vdsxe2x88x92Vds2/2].
In this relationship, Vgs, Vds and Vth respectively represent the gate-source voltage, the drain-source voltage and the threshold voltage. Beta depends on the production process and on the width-length ratio of the transistor. The output voltages of the voltage divider circuit depend on the process used (on account of Vth) and depend nonlinearly on the transistor dimensions. Therefore, the second requirement mentioned above is not fulfilled.
It is accordingly an object of the invention to provide a CMOS voltage divider that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which can be realized without passive components, such as resistors or capacitors, and can generate uniformly spaced output voltages from an applied input voltage while fulfilling the first through fourth requirements mentioned above.
With the foregoing and other objects in view there is provided, in accordance with the invention, a CMOS voltage divider. The CMOS voltage divider contains a first chain formed of series-connected first MOS transistors of a first conductivity type. Each of the first MOS transistors have identical geometrical dimensions and identical gate-source voltages. The first MOS transistors operate in a linear range of their characteristic curve and an input voltage to be divided is impressed between opposite ends of the first chain. The first MOS transistors furthermore have source terminals where voltage fractions of the input voltage can be picked off. A second chain formed of series-connected second MOS transistors of a second conductivity type being complementary to the first conductivity type is provided. The number of the second MOS transistors equals the number of the first MOS transistors. The second MOS transistors have the same geometrical dimensions in each case. The first MOS transistors are connected to the second MOS transistors such that each of the first MOS transistors of the first chain generates a gate-source bias voltage for a respective one of the second MOS transistors of the second chain and each of the second MOS transistors of the second chain generates the gate-source bias voltage for a respective one of the first MOS transistors of the first chain.
The transistors have the same size, that is to say that they are matched to one another, and therefore have identical gate-source voltages. Since they are connected in series with one another, their drain-source voltages are also identical. Moreover, the drain-source voltage is independent of process and temperature.
The invention achieves the above object by exclusively using mutually complementary MOS transistors of the N and P conductivity types. This reduces the area requirement, requires only an extremely small quiescent current and has only a very small output resistance, which, after all, is characteristic of CMOS technology. Furthermore, the output voltage depends only on the geometry of the circuit.
In accordance with the invention, the P-channel MOS transistors have drain terminals and gate terminals, and the N-channel MOS transistors have gate terminals and drain terminals. Each of the drain terminals of the N-channel MOS transistors is connected to a respective one of the gate terminals of the P-channel MOS transistors and each of the drain terminals of the P-channel MOS transistors is connected to a respective one of the gate terminals of the N-channel MOS transistors. The second chain has a source end to be connected to a first supply voltage and a drain end to be connected to a second supply voltage, and the following holds true:
VG greater than  greater than Vthreshold; VP=VG+VIN,
where:
Vthreshold denotes a maximum value of a threshold voltage of the N-channel and the P-channel MOS transistors;
VIN denotes the input voltage to be divided;
VP is the first supply voltage; and
VG is the second supply voltage.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a CMOS voltage divider, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.